Lightroom: Slide Show: It appears that images are not enlarged when rendering a JPG slide show. When exporting there is an option to not enlarge, but I could not find that option for the slideshow export. The Lightroom Queen suggested posting this here. I was using LR 5.6 or 5.7 when I first discovered this. I would almost call this a "problem" that it doesn't do this.

I have not checked this in LR6 because I need to delay until I accomplish a sync with another LR instance before I upgrade.

I worked around this by exporting with enlargement and then substituting the enlarged images for the originals in the slideshow.

I have this same problem in just playing a slide show within LR (CC 15.10.1 on Mac OS X 10.11.6), with any photo whose (edited) dimensions are less than the dimensions of the screen. The Slideshow module should upres the photos so that they fill the screen. Interestingly, this works properly outside the slideshow module (e.g., using the F key or using my 4K display as a secondary display (Window --> Secondary Display --> Show).

It is a huge hassle to export the photos at full resolution and then reimport them into LR.